1. Field of the Invention
The present invention relates to a liquid discharge apparatus that includes a liquid storage-discharge member having a liquid storage chamber for storing liquid and a liquid outlet for discharging the liquid from the liquid storage chamber to the outside of the liquid storage chamber, a vibration plate formed on the liquid storage-discharge member and a piezoelectric element having a lower electrode, a piezoelectric body and an upper electrode, the lower electrode, the piezoelectric body and the upper electrode being sequentially formed on the vibration plate.
2. Description of the Related Art
A piezoelectric element including a piezoelectric body and electrodes for applying an electric field to the piezoelectric body is used as an actuator that is mounted in a liquid discharge apparatus, such as an inkjet-type recording head, or the like. The piezoelectric body, which has a piezoelectric characteristic, expands or contracts as the intensity of the electric field applied thereto is increased or reduced. As piezoelectric material, perovskite-type oxide, such as a lead-zirconate-titanate-based oxide (PZT-based oxide), is well known. Such materials are ferroelectric materials, which spontaneously polarize without application of an electric field.
As illustrated in FIG. 7, an inkjet-type recording head according to the related art includes an ink nozzle (liquid storage-discharge member) 220 having an ink chamber (liquid storage chamber) 221 for storing ink, a vibration plate 230 formed on the ink nozzle 220 and a piezoelectric element 210 having a lower electrode 211, a piezoelectric body 213 and an upper electrode 214, for example. The lower electrode 211, the piezoelectric body 213 and the upper electrode 214 are sequentially formed on the vibration plate 230.
As illustrated in FIG. 7, when the ink chamber is pressured by piezoelectric deformation of the piezoelectric element and ink is discharged (jetted) from the ink nozzle, the edge portion (peripheral portion) of the piezoelectric element is constrained (restricted) by the ink nozzle and the central portion of the piezoelectric element slightly warps toward the ink chamber side. In this state, stress tends to be applied to the edge portion of the piezoelectric body (areas in circles in FIG. 7). Therefore, if the inkjet-type recording head is used for long time, there is a risk that the mechanical durability of the piezoelectric body is lost because the piezoelectric body is repeatedly driven in such a manner that displacement of the piezoelectric body occurs. For example, a crack is generated at the interface between a portion of the piezoelectric body, the portion to which high stress is applied, and a portion of the piezoelectric body, the portion to which high stress is not applied. In the specification of the present application, critical stress in general that may affect the durability of the apparatus by generation of a crack or the like is referred to as “fracture stress” in a broad sense (in the specification of the present application, the meaning of the term “fracture stress” is not limited to so-called fracture stress (fracture toughness) in a narrow sense, as a physical property of material).
To prevent such loss of durability, the piezoelectric body may be formed in an area that is smaller than the ink chamber so that no portion of the piezoelectric body is constrained by the ink nozzle. In that case, the piezoelectric element is supported only by the thin vibration plate. Therefore, there is a risk that the durability of the vibration plate is lost in long time. Hence, it is not desirable that the piezoelectric body is formed in such a manner.
An ultrasound actuator including a piezoelectric body having amorphous structure is disclosed in Japanese Unexamined Patent Publication No. 5(1993)-030763. Since grain boundary (crystal grain boundary) is not present in the amorphous structure, the amorphous structure can realize excellent mechanical durability of the ultrasound actuator.
Examples of piezoelectric strains are as follows:
(1) Ordinary piezoelectric strain, which is induced by application of an electric field
When the vector component of a spontaneous polarization axis and the direction of application of an electric field are the same, a piezoelectric material expands or contracts in the direction of application of the electric field as the intensity of the electric field applied thereto increases or decreases;
(2) Piezoelectric strain that is induced by non-180-degree reversible rotation of a polarization axis
The polarization axis rotates as the intensity of an electric field applied to the piezoelectric material increases or decreases;
(3) Piezoelectric strain utilizing a change in the volume of the piezoelectric material, the change being induced by phase transition of crystals
The phase transition occurs by increasing or reducing the intensity of an electric field applied to the piezoelectric material;
(4) Piezoelectric strain utilizing an engineered domain effect
The engineered domain effect is an effect that a larger strain is obtained by using a material having a property that phase transition is induced by application of an electric field and by forming crystal orientation structure including a ferroelectric phase, the direction of the crystal orientation of the crystal orientation structure being different from the direction of the spontaneous polarization axis. (When the engineered domain effect is utilized, the piezoelectric material may be driven in a condition in which phase transition can occur. Alternatively, the piezoelectric material may be driven within a range in which phase transition does not occur; and the like.
These piezoelectric strains (1) through (4) may be used alone or in combination to obtain a desirable piezoelectric strain. Further, in piezoelectric strains (1) through (4), if piezoelectric materials have crystal orientation structure that is appropriate for their respective strain generation principles, it is possible to obtain even larger piezoelectric strains. Therefore, it is desirable that a piezoelectric body has crystal orientation to achieve higher piezoelectric performance.
In Japanese Unexamined Patent Publication No. 2005-349714, polycrystallization of a piezoelectric material has been proposed. An amorphous film is formed using the piezoelectric material. Then, a portion of the amorphous film, the portion positioned on an ink chamber, is selectively annealed by a laser to polycrystallize the portion.
According to the method disclosed in Japanese Unexamined Patent Publication No. 2005-349714, it is possible to form polycrystalline structure in a main portion of the piezoelectric body, the portion positioned on the ink chamber, while maintaining the amorphous structure in the edge portion of the piezoelectric body. The main portion is a portion in which piezoelectric deformation should efficiently occur and the edge portion is a portion to which stress tends to be applied. However, in the method of polycrystallizing the piezoelectric material after temporarily forming amorphous structure, it is difficult to obtain high crystal orientation. Hence, it is difficult to achieve excellent piezoelectric performance.